This invention relates to apparatus for making concrete panels such as are used for erection of a building.
The desirability of constructing a building with precast concrete panels has been recognized for a long time but has not been practiced extensively. Both economic and technical problems have deferred widespread use. The major problem was economic in that mass production of panels or building components was not feasible using conventional methods of casting and curing concrete. Normally concrete must cure for at least 4 to 6 hours in a heated mold before demolding can be attempted. As a result of this long curing cycle, production facilities were occupied for long periods requiring costly duplication. An additional disadvantage of cast panels is the difficulty in obtaining dimensional accuracy, a good surface finish and high strength.
Some of the disadvantages of the foregoing conventional method have been overcome by making concrete articles in a mold which is inserted in a press to squeeze out water and thus reduce the time required before demolding. In this process, the mold is first filled with a concrete slurry having a water-to-cement ratio of about 0.7 to 1. Such concrete would have low strength if it were cured by just waiting for the normal hydration process to be completed. However, by pressing the concrete, sufficient water is expelled to reduce its water-to-cement ratio to the range of 0.3-0.4 to 1. With this lower ratio the concrete article attains sufficient strength to be handled immediately upon removal from the press, and its final strength will be considerably higher (almost 100%) than cast concrete with the same water, cement and aggregate ratio.
Reducing the water-to-cement ratio by pressing the concrete accelerates production but the process is not free of problems. First of all, presses which produce high total force on a relatively large area are required. Usually a unit pressure of more than several hundred pounds per square inch is required for adequate water removal and concrete density. For example, in order to produce panels of about 12 feet by 24 feet, presses having a total capacity of as much as 8,000 tons of pressure are required. The structural components of such presses must be substantially heavy.
Another difficulty in pressing building panels results from the necessity for door and window openings. When these panels are pressed there is a tendency for the platens of the press to deform in those regions where resistance is low because the presence of such an opening. This would result in a panel of nonuniform thickness. In some plants, total tonnage of the press is adjusted to compensate for openings in the molded article but in order to keep the platen deflections within reasonable limits, the platen had to be extremely stiff and a force absorbing guidance system was required, resulting in a considerably increase in weight of the press.
Another problem with prior presses is that the molds tend to bulge out at the sides due to lateral hydrostatic pressure which is developed while the concrete is being compacted. Molds of inordinate thickness had been used to resist this pressure. Molds that are sufficiently rigid to resist the lateral forces by themselves are unduly heavy and hard to handle.